1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an arrangement for use in irradiating laser light onto a recording medium having plural laminated recording layers.
2. Disclosure of Related Art
In recent years, the number of recording layers has been increasing, as the data capacity of an optical disc increases. The data capacity of a disc can be remarkably enhanced by forming plural recording layers in the one disc. In the case where recording layers are laminated, generally, two layers have been formed on one side of a disc. In recent years, however, a disc having three or more recording layers on one side thereof has been put into practical use in order to further increase the data capacity. An increase in the number of laminated recording layers enables to increase the data capacity of a disc. An increase in the number of laminated recording layers, however, may narrow the interval between the recording layers, and increase signal degradation resulting from an interlayer crosstalk.
Laminating recording layers weakens reflected light from a recording layer (a target recording layer) to be recorded/reproduced. Accordingly, incidence of unwanted reflected light (stray light) from a recording layer at an upper position or a lower position of the target recording layer into a photodetector may degrade a detection signal, and adversely affect focus servo control and tracking servo control. In view of this, in the case where a large number of recording layers are laminated, it is necessary to properly remove stray light, and stabilize a signal from a photodetector.
Japanese Unexamined Patent Publication No. 2009-211770 (corresponding to U.S. Patent Application Publication No. US2009/0225645A1) discloses a novel arrangement of an optical pickup device capable of properly removing stray light, in the case where a large number of recording layers are formed. With this arrangement, it is possible to form a rectangular area (signal light area) where only signal light exists, on a light receiving surface of a photodetector. Reflected light from a recording medium is irradiated at positions near vertex angles of the signal light area. By disposing sensors of a photodetector at the positions near the vertex angles of the signal light area, it is possible to suppress an influence on detection signals resulting from stray light.
In the optical pickup device thus constructed, an angular adjuster for changing the propagating direction of reflected light from a disc is used for irradiating signal light onto a signal light area, and the sensor is set to such a sufficiently large size as to receive signal light. However, if such an angular adjuster is configured of a step-type diffraction pattern, which is inexpensive, and plus first-order diffraction light caused by diffraction is irradiated onto the signal light area, minus first-order diffraction light which is not expected to be used may be entered into the sensor. Further, in this case, stray light of plus first-order diffraction light and stray light of minus first-order diffraction light may overlap and interfere with each other, which may form an interference fringe on the sensor.